The present invention relates to a mechanism for controlling pivoting of the rear axle of an industrial vehicle, such as a forklift. More specifically, the present invention pertains to a method and an apparatus for controlling an electromagnetic valve in a hydraulic circuit of the pivot controlling mechanism.
In a typical industrial vehicle such as a forklift, an axle for supporting the rear wheels pivots relative to a body frame in order to stabilize the body frame. However, if the vehicle is steered to change directions, pivoting of the rear axle may incline the body frame and thus destabilize the vehicle. The vehicle therefore has a mechanism for locking the rear axle when the vehicle is changing direction.
The axle locking mechanism includes a hydraulic circuit having a hydraulic cylinder. The hydraulic cylinder is located between the body frame and the rear axle and has two oil chambers. The oil chambers are connected to each other by an oil passage. An electromagnetic valve is located in the oil passage. Pivoting of the rear axle extends and retracts the hydraulic cylinder. In order to control the pivoting of the rear axle, the extension and retraction of the hydraulic cylinder are controlled. When the rear axle is allowed to pivot, the electromagnetic valve is open, allowing the oil chambers to communicate with each other, thereby allowing reciprocation of the hydraulic cylinder. On the other hand, when locking the rear axle, the electromagnetic valve disconnects the oil chambers from each other.
The electromagnetic valve is a two-way switch valve having a valve body that is moved between a connection position which allows the oil chambers to communicate with each other and a disconnection position for disconnecting the oil chambers. The position of the valve body is determined by the equilibrium of the force of a spring used to urge the valve body toward the disconnection position and the force of a solenoid that urges the valve body toward the connection position. When the solenoid is not excited, the force of the spring places the valve body in the disconnection position. When the solenoid is excited, the force of the solenoid surpasses the force of the spring and puts the valve body in the connection position. The electromagnetic valve is controlled by a controller, which also controls the pivoting of the rear axle.
When the forklift is carrying a relatively heavy load at a relatively high position, the controller does not excite the solenoid. Thus, the valve body is in the disconnection position and the rear axle is locked. When the forklift is carrying a relatively light load at a relatively low position, the controller excites the solenoid. This places the valve body in the connection position and allows the rear axle to pivot.
As described above, the solenoid does not place the valve body in the connection position unless it is excited. Therefore, when the solenoid malfunctions, the rear axle may be prevented from pivoting.
The electromagnetic valve is exposed to heat from the vehicle engine. The oil in the hydraulic circuit flows as the rear axle pivots. Frequent pivoting of the rear axle raises the temperature of the oil, which further raises the temperature of the circuit. Allowing the rear axle to continually pivot over an extended period of time, that is, exciting the solenoid for a long period of time raises the temperature of the solenoid. Consequently, the electromagnetic valve is excessively heated, which causes the solenoid to malfunction.